For all their advantages, diesel-fuelled engines have a significant disadvantage. When burned substantially in a diffusion combustion mode, diesel fuel generates high levels of some pollutants. Pollutants such as oxides of nitrogen (NOx) and particulate matter (PM) can be problematic. Cleaner burning gaseous fuels such as natural gas, hydrogen, ethane, propane, blends of gaseous fuels such as blends of natural gas and hydrogen, as well as others tend to expel fewer pollutants than diesel fuel when burned in an internal combustion engine. It has been determined that some gaseous fuels can also provide similar power output when directly injected at high pressure upon completion of a compression stroke or near the commencement of a power stroke in, for example, a four-stroke engine.
While providing emissions benefits, gaseous fuels tend to need some type of ignition assist to initiate combustion when used in an engine with diesel engine compression ratios. One common ignition assist is a pilot fuel. The pilot fuel is used to create an auto-ignitable charge that can be used to help initiate combustion of the gaseous fuel. The gaseous fuel can be directly injected, as noted above, or provided to the combustion chamber as a premixed fuel/air charge prior to combustion of the gaseous fuel. Such pilot ignited engines are considered one type of gaseous-fuelled compression ignition engine.
Gaseous-fuelled compression ignition engines can generally be controlled using engine maps that direct levers to control the start of combustion (SOC) based on the demands of the operator and the speed of the engine. For example, intake charge properties can be monitored and used to adjust SOC to target the release of combustion energy at a time appropriate to the engine speed and the load demands. However, controlling SOC fails to consider many aspects of engine operation important for controlling emissions and performance.
The heat release rate (HRR) seen during combustion of the fuel in a given cycle of the engine is an important determinant driving engine performance and emissions. Use of an ignition lever to control SOC alone fails to manage heat release rate once combustion has started. As such, advantages can be realized when heat release rate is controlled directly. In particular, an engine experiencing changes in the intake charge, which result in significant changes in the HRR, could benefit from a lever to adjust HRR based on a target HRR. This is the case where changes in the intake charge are unforeseen or desired for other purposes.
For example, when exhaust gas recirculation (EGR) is used to reduce NOx emissions significant cylinder-to-cylinder variations in EGR levels are possible, and transient deviations from the desired EGR rates may occur. Under these conditions, the variations in EGR levels can introduce undesirable changes in HRR. The changes in HRR can adversely impact performance of the engine. For example, higher concentrations of other pollutants such as PM and carbon monoxide (CO) may be generated. In general, limitations on EGR levels have been influenced by these undesirable changes in HRR.
Controlling HRR to a target HRR based on EGR levels allows increased levels of EGR and further reduction of NOx emissions while preserving engine performance and other emission targets. Therefore, advantages can be realized by adjusting for the influence of EGR on HRR.
Another example where control of HRR is important in gaseous-fuelled compression ignition engines arises from engines that employ premixed charge combustion ignition (PCCI). This includes engines that take advantage of a directly injected gaseous fuel that burns in a diffusion combustion mode (PCCI-DI) or not (PCCI). PCCI and PCCI-DI engines introduce an intake charge that can vary considerably over short- and long-term periods.
At least a portion of the energy for a PCCI-DI engine is provided by combustion of the premixed charge, which burns with fewer unwanted emissions than is the case for an equivalent amount of fuel burned in a diffusion combustion mode. The drawback, however, of premixing fuel prior to combustion, whether a directly injected quantity of main fuel is used or not, is a charge can be knock limited. That is, a premixed fuel/air charge may knock excessively if the SOC and HRR are not controlled, or the charge may not ignite at all resulting in a misfire. Variations in the intake charge (dictated by such things as methane number of the natural gas and other fuel properties, fuel/air ratio or intake charge temperature, by way of example) can vary considerably over the course of short and long-term periods. For example, a short term variation might be the result of a load transition, where the intake manifold temperature of the PCCI engine might, depending on hardware, take anywhere from 10 to 100 seconds to reach the desired value, by way of example. Long-term variations in intake charge properties may be the result of gaseous fuel composition changes over time, leading to different auto-ignition properties. Under both short term and long term cases, the HRR is strongly influence by the intake charge properties. Therefore, it is helpful to have a mechanism to control the resulting influence of such changes on HRR and thus have better control of engine performance and emissions.
As well as EGR levels and premixed charge properties, other strategies and conditions that influence HRR include the introduction of water into the combustion chamber for controlling emissions, operator demand put on the engine, and variations in ambient temperature, humidity and pressure, all of which benefit from use of a method to adjust to a target HRR.
HRR as both a variable for controlling engine performance and emissions and as an indicator of engine performance and emissions will, for the purposes of this application, be interchangeable with both actual HRR during the cycle of an engine and any variable used that is indicative of the actual HRR resulting from combustion of the fuel used. That is, HRR need not be a reference to an actual HRR trace for a given cycle of an engine. Herein, HRR will includes measures of variables such as cylinder pressure, exhaust gas properties (composition, temperature), intake charge properties (composition, temperature, etc.), and other variables indicative of actual HRR.
In this disclosure, diffusion combustion mode, stratified combustion mode and homogeneous combustion mode are referenced. Each provides an indication of combustion properties consistent with a generally unmixed charge of fuel and air wherein in combustion is thought to take place at the fuel/air interface, a partially mixed charge of fuel and air, and a premixed charge of fuel and air, respectively.
The present technique involves a method of adjusting HRR, both dependent upon and independent of SOC, within a gaseous-fuelled internal combustion engine.